1. Field of the Invention
The present invention relates to an endoscope capable of performing front-view observation and lateral-view observation.
2. Description of the Related Art
In recent years, endoscopes provided with illumination means and observation means on a distal end side of an insertion portion have been widely used in the medical field and the like.
An endoscope is sometimes used for examining an internal wall of a tubular organ by inserting an insertion portion of the endoscope into the tubular organ. In order to facilitate such an examination, endoscopes have been developed, which have a lateral field of view in which the observation field of view is in the lateral-surface direction (lateral side) of the insertion portion, in addition to a front field of view in which the observation field of view is on the forward side of the insertion portion along the insertion direction or in the axial direction of the insertion portion.
For example, Japanese Patent Application Laid-Open Publication No. 2004-329700, as a first prior art example, discloses an endoscope having both a front observation field of view and a lateral observation field of view.
Japanese Patent Application Laid-Open Publication No. 2008-309860, as a second prior art example, discloses an optical system of an endoscope including an optical system which is rotationally symmetrical with respect to the central axis and includes a front-view optical path for picking up an image of an object in the central axis direction, and a wide-angle lateral-view optical path (lateral-view observation portion) for forming an omnidirectional (lateral-side entire circumferential) annular image outside of a circular image through the front-view optical path on the same one image pickup device by performing reflection at least twice in the annular optical element and using a part of the front-view optical path.